1. Field of the Invention
This invention relates to injection molding procedures in which multiple cavities are simultaneously filled with plastic material. The invention is particularly concerned with situations where it is desired to mold a number of parts of dissimilar size in the same mold, or to use several nozzles for a single cavity, and also to adjust the mold gate opening at each shut-off bushing to create optimum flow conditions.
2. Description of the Prior Art
It is known to provide individual gate control on molding machines themselves, as contrasted with mold shut-off bushings. However, this means that the mold has to be constructed to fit the particular molding machine, greatly reducing the versatility of the construction and increasing manufacturing costs where a number of different molded parts are to be made.
Kelly U.S. Pat. No. 2,770,011 shows an injection molding machine in which a timer controls a fluid motor which operates retractable wedges so that gate pins for a plurality of mold cavities may be simultaneously retracted. However, there is no suggestion in Kelly of individual timing and control of gate pins nor of positive retraction thereof independently of material pressure. In Kelly, furthermore, the gate pins are not adjustable and the material outlets to the cavities are therefore of fixed size. The Kelly construction therefore does not permit individual variation of individual flow conditions to each cavity and it would not be feasible to use this construction for the simultaneous filling of cavities of dissimilar sizes.
Natkins U.S. Pat. No. 3,491,408 shows a distribution manifold carrying a number of spring-urged valve members which are retracted by material pressure, the extent of opening of the valve members being individually adjustable. Aside from the fact that this stop mechanism is shown on an injection molding machine rather than on the mold itself, it suffers from the drawback that injection pressure is used to activate the gates. This often causes material to "explode" into the cavity, inducing extra shear and creating additional heat that may affect the plastic.
Bielfeldt et al U.S. Pat. No. 3,847,525 shows an injection molding apparatus utilizing a distributing valve for sequential delivery to individual mold cavities, with rams controlled by double-acting pistons. This patent however has numerous drawbacks as compared with the present invention which will become apparent from the following description.
Barrie U.S. Pat. No. 3,909,169 and British Pat. No. 1,056,861 show injection molding systems having general pertinency to the present invention. Strauss U.S. Pat. No. 2,828,507 and Angell U.S. Pat. No. 3,436,446 show gate pins controlled by double acting fluid motors. These patents however are not believed to teach the present invention.